1. The Field of the Invention
This invention relates to the field of printed feature manufacturing, such as integrated circuit manufacturing. In particular, this invention relates to automatically identifying evaluation points where errors are computed and analyzed to achieve improved agreement between a design layout and an actual printed feature.
2. Description of Related Art
To fabricate an integrated circuit (IC), engineers first use a logical electronic design automation (EDA) tool, also called a functional EDA tool, to create a schematic design, such as a schematic circuit design consisting of symbols representing individual devices coupled together to perform a certain function or set of functions. Such tools are available from CADENCE DESIGN SYSTEMS and from SYNOPSYS. The schematic design must be translated into a representation of the actual physical arrangement of materials upon completion, called a design layout. The design layout uses a physical EDA tool, such as those available from CADENCE and AVANT!. If materials must be arranged in multiple layers, as is typical for an IC, the design layout includes several design layers.
After the arrangement of materials by layer is designed, a fabrication process is used to actually form material on each layer. That process includes a photo-lithographic process using a mask having opaque and transparent regions that causes light to fall on photosensitive material in a desired pattern. After light is shined through the mask onto the photosensitive material, the light-sensitive material is subjected to a developing process to remove those portions exposed to light (or, alternatively, remove those portions not exposed to light). Etching, deposition, diffusion, or some other material altering process is then performed on the patterned layer until a particular material is formed with the desired pattern in the particular layer. The result of the process is some arrangement of material in each of one or more layers, here called printed features layers.
Because of the characteristics of light in photolithographic equipment, and because of the properties of the material altering processes employed, the pattern of transparent and opaque areas on the mask is not the same as the pattern of materials on the printed layer. A mask design process is used, therefore, after the physical EDA process and before the fabrication process, to generate one or more mask layouts that differ from the design layers. When formed into one or more masks and used in a set of photolithographic processes and material altering processes, these mask layouts produce a printed features layer as close as possible to the design layer.
The particular size of a feature that a design calls for is the feature""s critical dimension. The resolution for the fabrication process corresponds to the minimum sized feature that the photolithographic process and the material processes can repeatably form on a substrate, such as a silicon wafer. As the critical dimensions of the features on the design layers become smaller and approach the resolution of the fabrication process, the consistency between the mask and the printed features layer is significantly reduced. Specifically, it is observed that differences in the pattern of printed features from the mask depend upon the size and shape of the features on the mask and the proximity of the features to one another on the mask. Such differences are called proximity effects.
Some causes of proximity effects are optical proximity effects, such as diffraction of light through the apertures of the optical systems and the patterns of circuits that resemble optical gratings. Optical proximity effects also include underexposure of concave corners (inside corners with interior angles greater than 180 degrees) and overexposure of convex corners (outside corners with interior angles less than 180 degrees), where the polygon represents opaque regions, and different exposures of small features compared to large features projected from the same mask. Other causes of proximity effects are non-optical proximity effects, such as sensitivity of feature size and shape to angle of attack from etching plasmas or deposition by sputtering during the material altering processes, which cause features to have shapes and sizes that have decayed from or accumulated onto their designed shapes and sizes.
In attempts to compensate for proximity effects, the mask layouts can be modified. To illustrate, FIG. 1A shows mask items 170, such as windows or opaque areas on a mask, represented by edges 171 on one or more polygons, and some printed features 180 with exaggerated proximity effects. FIG. 1A does not represent an actual example, but is provided simply to illustrate the concepts of proximity effects and mask modifications to attempt to correct for proximity effects. To make apparent the illustrated proximity effects, the projection of the original polygons 171 is shown as a fine line around the printed features 180. Note that the printed features 180 includes a spurious connection feature 182, an edge 185 entirely displaced inside the corresponding edge of the original outline 171, and an end line 183 completely inside the outline of the original items 171.
FIG. 1B illustrates various ways mask items 190 are modified to correct for such effects. FIG. 1B is not a particular example of a particular set of corrections that actually mitigate the proximity effects illustrated in FIG. 1A. The corrections available include hammerheads 192 (i.e. hammerheads 192a and 192b) added to ends of items to compensate for overexposure of the entire end line of a feature. Also shown are biases 196 (i.e. biases 196a and 196b) applied along portions of a straight edge of a feature. A negative bias like 196 represents a portion of an opaque area made transparent (or a portion of a window made opaque). In this case the negative bias reduces the size of the item on a mask to avoid generating the spurious feature 182 of FIG. 1A. Also shown are assist features 194 (i.e. assist features 194a and 194b), which are separate items smaller than the resolution of the photolithographic process and thus too small to be formed in a photoresist layer, but which are sufficiently large to effect diffraction patterns that influence larger nearby features. The assist features 194 are intended to move the edge 185 of the printed features 180 in FIG. 1A toward the outline 171 of the original mask items 170. Also shown is a sub-resolution serif 193 of extra opaque material to compensate for overexposure at convex corners of opaque areas, and an anti-serif 197 indicating where opaque material, if any, is removed to compensate for underexposure at concave corners of opaque polygons. These corrections are listed to illustrate the concepts of correcting a mask to compensate for proximity effects. The illustrated corrections do not necessarily correct the depicted features.
In rule-based proximity corrections, corrections such as the serifs and biases of a predetermined size are automatically placed at corners and edges of fabrication layout shapes like the mask shapes 170. Other rules may include adding assist shapes to a mask near desired features from the design layout and adding hammerheads to short end lines of the desired features. Experience of engineers accumulated through trial and error can be expressed as rules, and applied during the fabrication design process. For example, a rule may be expressed as follows: if a feature is isolated, then widen the opaque region in the mask by a particular amount to compensate for expected overexposure, so that the feature prints properly.
The experience captured in rule-based corrections is garnered by going through the fabrication process repeatedly with different mask layouts, making adjustments and observing the results. However, this process consumes time and manufacturing capacity. Even if such experimental, rule-forming runs are made, the resulting rules often do not give satisfactory results as the features become more complicated, or become smaller, or interact in more confined areas, or involve any combination of these effects.
Because rule based corrections are often imprecise and time consuming to revise and test, a proximity effects model is often developed and used to predict the effects of a change in the mask layout with more precision and with fewer off line fabrication runs.
A proximity effects model is typically built for a particular suite of equipment and equipment settings assembled to perform the fabrication process. The model is often built by performing the fabrication process one or a few times with test patterns on one or more mask layouts, observing the actual features printed (for example with a scanning electron micrograph), and fitting a set of equations or matrix transformations that most nearly reproduce the locations of edges of features actually printed as output when the test pattern is provided as input. The output of the proximity effects model is often expressed as an optical intensity. Other proximity effects models provide a calibrated output that includes a variable threshold as well as an optical intensity. Some proximity effects models provide a calibrated output which indicates a printed edge location relative to a particular optical intensity as a spatial deviation. Thus some model outputs can take on values that vary practically continuously between some minimum value and some maximum value.
Once a proximity effects model is produced, it can be used in the fabrication design process. For example, the proximity effects model is run with a proposed mask layout to produce a predicted printed features layer. The predicted printed features layer is compared to the design layer and the differences are analyzed. Based on the differences, modifications to the proposed mask layout are made, such as by adding or removing serifs 193, or adding or removing anti-serifs 197. After making these corrections to the proposed mask layout, a final mask layout is generated that is used in the fabrication process.
A proximity effects model typically produces predictions that lead to corrections that are more accurate than rule based corrections. However, a proximity effects model consumes computational resources by an amount that is related to the number of points of interest where amplitudes are computed. For typical mask layouts, the number of points where the model could be run is large, and the computation time is prohibitive Therefore it is typical to run the proximity effects model at selected evaluation points located on the edges of features, to determine the correction needed, if any, at each evaluation point, and then to apply that correction to all the points on a segment of the edge in the vicinity of the evaluation point.
If too many evaluation points are selected, the model runs too slowly and the correction procedure takes too long. If too few evaluation points are selected, the model may not detect critical locations where corrections are necessary. Another undesirable effect of too few evaluation points is that segments can become too large, so that even if a correction is accurate for the evaluation point, the correction is applied to too much of the edge. This causes undesirable changes on the edge away from the evaluation point.
What are needed are new ways to select evaluation points and define their vicinity for the mask layouts, so that a reasonable number of comparisons can be made and so that effective modifications to the mask layouts can be suggested.
According to techniques of the present invention, edges of polygons in a mask layout are dissected into segments defined by dissection points, with each segment having an evaluation point, according to modified dissection rules. These techniques allow the spacing of evaluation points and dissection points to be automatically adapted to portions of each edge where changes in the proposed mask layout are most likely needed. According to these techniques, dissection points are closer together where proximity effects are more significant and are farther apart where proximity effects are less significant. Thus unnecessary evaluation points are eliminated and the analysis process is speeded up, while still retaining needed evaluation points.
In one aspect of these techniques, all dissection points can be derived from just a few easily defined parameters. The first parameter is a corner dissection segment length, Lcor, used as a target length to compute dissection points for corner segments that include at least one vertex of the polygon when the length of the edge allows. The second parameter is a detail dissection segment length, Ldet, used as a target length to derive dissection points for segments along portions of an edge where changes in proximity effects are expected to be significant, such as adjacent to corner segments and around projection points when the length of the edge allows. A projection point is a location on an edge where a vertex of another edge comes close enough to introduce sharp changes in the proximity effects. A halo distance is a parameter that determines whether a vertex is close enough to introduce an important effect A maximum dissection segment length, Lmax, by definition larger than either Lcor or Ldet, is used as a target length to further dissect long portions of the edge that would otherwise not be dissected, such as away from corners and projections points. Extra precision is achieved if a true-gate extension length parameter Lext is used on edges of shifters forming true-gates. The dissection parameters Lcor, Ldet, Lmax, halo and Lext are pre-set and accessed as needed.
In one aspect of the disclosed techniques, a profile of amplitudes from a proximity effects model along an edge of a feature in a design layout is used to dissect the edge. This amplitude profile reflects the level of proximity effects for the given layout and fabrication process. By using this information, a better dissection of the edge is achieved in which the segment length between successive dissection points depends on the magnitude of the proximity effects. For example, on a portion of an edge where the magnitude of the proximity effects is larger than another portion, the segment length is made longer. This kind of dissection avoids making the segment length too short. A segment that is too short, for example in a corner, may result in a very large correction, and may prevent the overall corrections for the layout from converging.
In another aspect, the dissection parameters are derived from the segment lengths based on profiles of amplitudes. According to the modified dissection rules, each evaluation point is placed along a segment at a predetermined fraction of the distance from one dissection point to the other defining each segment, where the predetermined fraction depends on the type of segment. For example, an evaluation point is placed closer to the non-vertex dissection point in a corner segment; and is placed midway between dissection points in other types of segments. Also according to the modified dissection rules, segments are located on polygon edges according to the type of edge, such as whether the edge is a line end, a turn end, or a longer edge. These edge types are deduced from the length of the edge compared to the dissection parameters and whether one or both vertices of the edge are concave or convex corners.
Specifically, according to some aspects of the invention, techniques for correcting proximity effects in a proposed layout corresponding to a design layout for a printed features layer include selecting from among all edges of all polygons in the proposed layout a subset of edges for which proximity corrections are desirable. The subset of edges includes less than all the edges. Evaluation points are established only for the subset of edges. Corrections are determined for at least portions of the subset of edges based on an analysis performed at the evaluation points.
According to other aspects, techniques for correcting proximity effects associated with an edge corresponding to a design layout include establishing a projection point on a first edge corresponding to the design layout based on whether a vertex of a second edge corresponding to the design layout is within a halo distance. The halo distance is associated with significant proximity effects on one edge from a vertex of another edge. The first edge is divided into segments based on the projection point and characteristics of the first edge. It is then determined how to correct the first edge for proximity effects based on the segments.
According to other aspects, techniques for correcting proximity effects associated with an edge corresponding to a design layout include establishing a projection point on a first edge. An evaluation point is determined for the first edge based on the projection point and characteristics of the first edge. It is then determined how to correct at least a portion of the edge for proximity effects based on an analysis at the evaluation point.
According to one embodiment of this aspect of the invention, if the vertex of the second edge is within the halo distance of the first edge, then establishing a projection point includes placing the projection point on the first edge closest to the vertex.
According to another embodiment of this aspect of the invention, determining the evaluation point includes establishing on the first edge two evaluation points spaced apart at least by a target length for detail segments. The two evaluation points straddle the projection point.
According to another embodiment of this aspect of the invention, before establishing a projection point, the halo distance is determined. This includes executing a proximity effects model at several points along a test edge to produce a profile of model amplitudes. A variation in amplitude is determined that is associated with a test vertex on a different test edge. A distance between the test edge and the test vertex is associated with the variation in amplitude. The proximity model is run, the variation is determined, and the distance is associated with the variation for several test edges. Then the halo distance is set based on a distribution of distances associated with variations for the test edges.
According to another embodiment of this aspect of the invention, a computer system for correcting proximity effects associated with an edge corresponding to a design layout includes a computer readable medium carrying data representing the edge, the halo distance, and at least a portion of the design layout. The design layout corresponds to a portion of an integrated circuit. One or more processors are coupled to the computer readable medium. The processors are configured to perform the following steps. A projection point is established on a first edge corresponding to the design layout based on whether a vertex of a second edge is within the halo distance. An evaluation point for the first edge is determined based on the projection point and characteristics of the first edge. A proximity effects model is executed for the evaluation point to produce a model amplitude. A correction is determined for at least a portion of the first edge including the evaluation point based on an analysis of the model amplitude. The correction is then applied to that portion of the first edge.
According to another embodiment of this aspect of the invention, a device is fabricating using a mask for fabricating a printed features layer that includes an opaque region having a segment corrected for proximity effects. The segment corresponds to at least one portion of a first edge in a design layer for the printed features layer. The segment is displaced from the corresponding portion in the design layer by a correction distance. The correction distance is based on analysis of an amplitude output by a proximity effects model at an evaluation point on the corresponding portion. The evaluation point is based on a projection point and characteristics of the edge. The projection point is established based on whether a vertex of a second edge corresponding to the design layout is within the halo distance of the first edge.